The spatial resolution of radar systems typically depends on the beamwidth, the size of the radar antennas, and the distance between the radar and the target. Consequently, there is typically a tradeoff between these factors and achievable resolution in radar sensing.
Traditionally, spatial resolution could be increased by increasing the antenna size. For example, as the antenna aperture size is increased, the beamwidth of the antenna (i.e., 3 dB attenuation with respect to the peak) decreases. An approximate expression for the inverse relation between the antenna size (D) and the 3 dB beamwidth (BW) for a horn antenna is BW=70λ/D, where λ is the wavelength. As a result of the decreased beamwidth, the spot size on the target decreases as well, enabling the detection of smaller features on the target. The enlargement of the antenna size needs to be done on either or both the transmitter's and the receiver's antennas of the radar in order to improve the radar resolution. However, the use of large antennas, such as large reflector dishes or arrays of antenna elements, can increase the system cost, size, and weight. In addition, it may be necessary to mechanically rotate or move the antenna or target in order to observe different features on the target such that the relevant area on the target would be illuminated. Larger antennas can be made by constructing phased arrays that use, for example, digital beamforming, or multi-static methods. However, phased array antennas add complexity in the form of individual feeds, transceivers, and phase shifters, and/or processing sections.
Spatial resolution can also be increased by moving the radar system closer to the target. FIG. 1 shows the effect of antenna size, target distance, and beamwidth on the spot size on the target. For a given antenna size (D) and 3 dB beamwidth (BW), the spot size on the target (X) depends on the distance (R) between the target and the radar antenna. The spot size can be approximated as X=2Rtan [λ/(2D)], where λ is the wavelength. However, moving the antenna closer to the target is not always a practical approach, as one cannot always control the distance between the radar and the target. In addition, the spot size does not decrease with the radar-target distance when the distance becomes shorter than the far-field distance D2/λ. In fact, the minimum spot size close to the antenna is of the order of the antenna aperture size itself, as is shown in FIG. 2.
Super-resolution techniques can also be used to improve the resolution of radar systems. Super-resolution techniques, however, may require increased processing power and sophisticated algorithms. In addition, many super-resolution techniques require a high signal-to-noise ratio for efficient processing. Other techniques, such as monopulse can also be used to increase the resolution.
What is needed is a radar system with resolution independent of the range to the target.